BFFs Luke Prael and Sterling Jerins hang out in the theater district.Zandy Mangold

They’ve been friends forever, doing what young Upper East Siders do: swim, shop, spy on their doormen.

Who’d have guessed they’d wind up trying to stab each other with a pair of scissors?

Happily, Luke Prael and Sterling Jerins were only faking that last activity in “Boarding School,” the horror film that opens Friday. It’s the first flick the teen actors made together — the first professional one, that is.

“As children, we always made short movies, but they were terrible,” says 14-year-old Sterling, whose credits include “World War Z,” two “Conjuring” films and “Divorce,” the HBO series.

Luke, the 15-year-old star of this summer’s acclaimed film “Eighth Grade,” remembers the key role he played in one of those amateur shorts, “The Haunted Foot.”

“I always thought how cool it would be to do a [real] movie together,” Luke says.

Luke Prael and Sterling Jerins in a scene from “Boarding School.”Momentum Pictures

Over lunch at Sardi’s, they tell The Post they didn’t know they were up for the same film until they bumped into each other at a callback. “Boarding School” director Boaz Yakin and his team didn’t even know they knew each other.

As their mothers will tell you, they’ve been close since birth, and not just because they live two blocks apart. Their older sisters act, as do Luke’s dad and Sterling’s mom, who coaches her.

They shot “Boarding School” in a splendid but spooky mansion in Riverdale. The R-rated flick is full of frights: wicked stepparents, sadistic schoolteachers, a demonic doll and the occasional Nazi. Nevertheless, they say, it didn’t give them nightmares, though Sterling says the doll made her “sort of freak out.”

But she loved shooting the fight scene. Working with a choreographer, it took three days to film. “It was like, ‘Scissors! Punch, punch, punch,’ almost like a dance,” she says. “I really enjoyed it.”

Luke, not so much. Maybe because he was wearing a dress, which has something to do with his character’s channeling a dead grandmother.

Sterling shakes her head. “Luke’s a manly man,” she says. “The fact that he can do it shows how strong he can be!”

‘As children, we always made short movies, but they were terrible.’

Though she’s acted alongside her fair share of stars — Sarah Jessica Parker and Thomas Hayden Church play her parents on “Divorce” — she was stunned to find “Gossip Girl” actor Robert John Burke playing her “Boarding School” dad.

“OMG, Luke!” she whispered. “It’s Bart Bass!” Still, she says, she knows what it’s like to be on the other side of star-struck, having been stopped in an Urban Outfitters dressing room by a fan saying, “Sorry to bother you, but I love you in ‘Divorce.’”

Social media can be hard on privacy, they say, which is why they keep separate Instagram accounts: one for the public and the people they work with, the other for family and friends.

And yes, say the actors, whose salaries go into their college funds — minus the money Luke spends on skateboarding duds from Supreme — they’d like to keep performing. They tell other aspiring actors not to let rejection get them down.

“You can always take something from an audition,” Luke says. “Try your best, and learn from it.”

As far as Sterling’s concerned, her pal has learned a lot already.

“The fact that he can play all these different roles is crazy!” she says. “He’s able to tap into all these different characters and make them himself.”